The overexpression of procaspase-3 (PC-3) in diverse tumor types presents an exploitable weakness in cancer and an opportunity for selective induction of apoptotic death in cancer cells. This proposal is a renewal of R01-CA120439, whose major goals included the investigation of the potential of the PC-3 activator PAC-1 in the treatment of glioblastoma (GBM) and metastatic osteosarcoma. These past four years have been very successful and as a result of these studies PAC-1 is now in a single-agent Phase 1 clinical trial (for late stage human cancer patients) and a combination clinical trial (for human glioblastoma patients), with other trials to being in 2018. Our work has shown that PAC-1 treatment, through activation of PC-3 to caspase-3, can induce cleavage of key caspase-3 substrates in cancer cells, leading to pronounced synergy with certain anticancer drugs. In the proposed work we will identify the most promising PAC-1/drug combinations for the treatment of Grade II/III meningioma, taking advantage of a key feature of PAC-1 (its blood-brain barrier penetrance) and utilizing our novel translational pathway (evaluation in pet dogs with meningioma). We have also found that PAC-1 markedly synergizes with drugs that hit targets in the MAPK pathway, due to the ability of PAC-1 to induce cleavage and inactivation of MEK1 and MEK2. This discovery will be explored through investigation of PAC-1 combinations in non-small cell lung cancer models, utilizing mouse models of metastases to the CNS. Finally, based on known caspase-3 substrates and preliminary data, we hypothesize that PAC-1 will have considerable mechanism-based synergy with immune checkpoint inhibitors through the facile cleavage and inactivation of the protein MLH1 by caspase- 3. Logical extension of this data would suggest that such mechanism-based synergy with PAC-1 could lead to immune checkpoint inhibitors being active in a much higher percentage of cancer patients, an exciting possibility. This is a hypothesis-driven proposal with high potential for fundamental and translational impact.